Biogeochemistry of dissolved organic matter (DOM), 210Pb, and 210Po in the East Sea

Abstract

The East Sea is a semi-enclosed marginal sea, which includes the various oceanographic characteristics such as the cold water, warm water, sub-polar front, and eddies in the surface ocean. In particular, the deep water of this sea is formed by the thermohaline circulation. Therefore, globally important biogeochemical processes can be studied very effectively in this miniature of the global ocean. In order to determine various biogeochemical processes, vertical and horizontal distributions of dissolved oxygen (DO), photosynthetic pigments, nutrients, dissolved organic matter (DOM), 210Pb, and 210Po were determined from 2001 to 2009 in the East Sea. The average concentrations of dissolved inorganic nitrogen (DIN) and phosphate (DIP) were consistently 9.8 ± 6.2 μM and 0.80 ± 0.40 μM in the surface layer (0–200 m), and 23 ± 2 μM and 1.81 ± 0.20 μM in the deep layer (200–3500 m). The DIN:DIP ratios in the East Sea were lower than 10 in the mixed layer, with a constant ratio of 13 in the deep ocean. Especially, the DIN:DIP ratios in the deep ocean were considerably lower than those in major oceans. A simple mass-balance box model shows that the combined effects of physical (the rapid ventilation of the surface water with a low DIN:DIP ratio of <10) and biological (remineralization of sinking organic matter) processes could result in low DIN:DIP ratios over the deep water column of the East Sea. The concentrations of dissolved organic carbon (DOC, 60–83 µM) and dissolved organic phosphate (DOP, 0.1–0.4 µM) in the surface layer of the East Sea fell into a range similar to those found in the major oceans, whereas the concentration of dissolved organic nitrogen (DON, 2–7 µM) was lower than that in the Pacific Ocean (7–13 µM). The dissolved (<10 kD), colloidal (10 kD–0.45 µm), and particulate (>0.45 µm) 210Pb activities in the surface layer of the East Sea were in the range of 2.0–10, 2.3–13, and 0.9–3.7 dpm/100L, respectively. The dissolved, colloidal, and particulate 210Po activities in the surface layer of the East Sea were in the range of 0.4–2.7, 0.1–4, and 1.5–4.6 dpm/100L, respectively. The residence times of dissolved, colloidal, and particulate 210Po were 34 ± 14, 57 ± 18, and 194 ± 72 days, respectively. The lower DON concentrations, higher DOC:DON ratios, and lower DON:DOP ratios in the East Sea, relative to the major oceans, together with a strong negative correlation between the concentrations DON and zeaxanthin, biomarker of cyanobacteria, suggest that DON is preferentially taken up by cyanobacteria in N-limited environment. In this connection, cyanobacteria (20–65%) dominated the phytoplankton community in the East Sea on the basis of photosynthetic pigment analysis. Thus, under DIN-limited condition, the C:N:P stoichiometry of DOM in the East Sea seems to be controlled by cyanobacteria. The residence time of colloidal 210Po was several-fold longer than typical turnover times (<10 days) of high-molecular-weight DOC (HMW-DOC). This result suggests that 210Po turns over several times through HMW-DOC, perhaps together with other bio-reactive elements, before settling down from the upper ocean.